Method of controlling uplink synchronization state at a user equipment in a mobile communication system

ABSTRACT

A method of controlling uplink synchronization state at a user equipment in a mobile communication system is disclosed. The method of controlling an uplink synchronization state at a user equipment in a mobile communication system comprises receiving control information associated with controlling uplink synchronization state of the user equipment from a network, releasing uplink resources allocated from the network if the uplink synchronization state is shifted from a synchronization state to an asynchronous state based on the control information.

TECHNICAL FIELD

The present invention relates to a mobile communication system, and moreparticularly, to a method of controlling uplink synchronization state ata user equipment in a mobile communication system.

BACKGROUND ART

In a mobile communication system which uses multiple carriers, such asan orthogonal frequency division multiple access (OFDMA) or a singlecarrier-frequency division multiple access (SC-FDMA), radio resourcesare a set of continuous sub-carriers and are defined by a time-frequencyregion on a two-dimensional sphere. A time-frequency region in the OFDMor OFDMA scheme is a rectangular form sectioned by time and sub-carriercoordinates. In other words, one time-frequency region could be arectangular form sectioned by at least one symbol on a time axis and aplurality of sub-carriers on a frequency axis. Such a time-frequencyregion can be allocated to an uplink for a specific user equipment (UE),or a base station can transmit the time-frequency region to a specificuser equipment in a downlink. In order to define such a time-frequencyregion on the two-dimensional sphere, the number of OFDM symbols on thetime region and the number of continuous sub-carriers on the frequencyregion should be given, wherein the continuous sub-carriers start from apoint having an offset from a reference point.

FIG. 1 illustrates an example of a structure of a physical channel usedin a multiple carrier system according to the related art. In FIG. 1, asub-frame comprises an L1/L2 control information transmission region(hatching part) and a data transmission region (non-hatching part).

Referring to FIG. 1, a physical channel includes a plurality ofsub-frames on the time axis and a plurality of sub-carriers on thefrequency axis, wherein one sub-frame includes a plurality of symbols onthe time axis. One sub-frame includes a plurality of resource blocks(RBs), each of which includes a plurality of symbols and a plurality ofsub-carriers. Also, each sub-frame can use specific sub-carriers ofspecific symbols (for example, first symbols) for a physical downlinkcontrol channel (PDCCH), i.e., L1/L2 control channel. One sub-frame hasa length of 0.5 ms, and transmission time interval (TTI) that is a unittime of data transmission has a length of 1 ms corresponding to twosub-frames.

In a mobile communication system, radio resources of one cell includesuplink radio resources and downlink radio resources. The base stationserves to allocate and control downlink radio resources and uplink radioresources of a cell. In other words, the base station determines whatuser equipment uses what kinds of radio resources and when thecorresponding user equipment uses the corresponding radio resources. Forexample, the base station can determine that frequencies 100 Mhz and 101Mhz are allocated to a first user equipment for downlink datatransmission for 0.2 seconds after 3.2 seconds. The base stationnotifies the first user equipment of the determined fact so that thefirst user equipment can receive downlink data. Likewise, the basestation determines what user equipment transmits uplink data through anuplink using how many radio resources, and also determines when thecorresponding user equipment transmits uplink data. Also, the basestation notifies the corresponding user equipment of the determined factso that the corresponding user equipment can transmit uplink data.

In the related art, a specific user equipment has continued to usespecific radio resources while call connection is being maintained.However, such a structure is inefficient in a recent communicationsystem, which provides many services based on IP packets. This isbecause that most of packet services do not generate packetscontinuously for call connection time. Namely, packets may betransmitted for a specific interval but no packets may be transmittedfor another specific interval. It is not efficient that radio resourcesare continuously allocated to a specific user equipment for callconnection in the above packet-based system. In order to solve thisproblem, a recent mobile communication system uses a method ofdynamically allocating radio resources to the user equipment if the userequipment needs the radio resources or only if there are data to betransmitted to the user equipment.

In the system which uses OFDM or SC-FDMA system, a frequency band isdivided into bands of a constant size and each band is allocated toseveral user equipments. In this case, the base station may not receivedata transmitted to an uplink through each frequency band due tointerference of data transmitted from another band. In order to avoidthis, synchronization in transmission time between the respective userequipments is necessarily required. In other words, when the first userequipment and the second user equipment are scheduled to transmit uplinkdata for a specific time interval, the time when the data transmittedfrom the first user equipment arrive in the base station should beidentical with the time when the data transmitted from the second userequipment arrive in the base station. At this time, if there is a littledifference in timings when the data transmitted from the first andsecond user equipments arrive in the base station, the data transmittedfrom the first and second user equipments cannot be recovered in thebase station successfully.

Accordingly, in the system which uses OFDM or SC-FDMA system, uplinksynchronization of the respective user equipments is necessarilyrequired. To maintain uplink synchronization, various methods are used.One of the various methods is a synchronization method based on a randomaccess procedure through a random access channel (RACH).

The random access procedure will now be described in brief The userequipment, which is in a non-synchronization state, transmits a bitstream, which is previously determined, i.e., signature, to the basestation. The base station detects the signature, and calculates how slowdata transmission of the user equipment should be performed or how fastdata transmission of the user equipment should be performed, so as toreach uplink synchronization based on the detected signal. The basestation reports the calculated result to the user equipment. The userequipment adjusts the transmission time of uplink data based on thecalculated result and then obtains uplink synchronization.

Hereinafter, radio resource control (RRC) state of the user equipmentand its RRC connection method will be described. The RRC state meanswhether RRC layer of the user equipment is logically connected with RRClayer of the network. The RRC state is called RRC connected state if theRRC layer of the user equipment is logically connected with the RRClayer of the network. On the other hand, the RRC state is called RRCidle state if not so. The network can identify the presence of the userequipment of the RRC connected state in a cell unit due to the presenceof the RRC connection. Accordingly, the network can effectively controlthe user equipment. By contrast, the network cannot identify the userequipment of the RRC idle state, and a core network manages the userequipment of the idle state in a unit of a tracking area which is alocal unit greater than the cell. In other words, in case of the userequipment of the RRC idle state, its presence is identified in a greatlocal unit. The user equipment should be shifted to the RRC connectedstate to obtain a typical mobile communication service such as voice ordata.

When a user first turns on the power of the user equipment, the userequipment searches a proper cell and then stays in the correspondingcell in the RRC idle state. The user equipment, which stays in the RRCidle state, performs RRC connection with the RRC layer of the networkthrough an RRC connection procedure if the RRC connection is required,and is shifted to the RRC connected state. The user equipment, which isin the RRC idle state, needs the RRC connection in case of variousexamples. For example, the user equipment which is in the RRC idle stateneeds the RRC connection if uplink data transmission is needed due totrying calling of the user or if a response message transmission to apaging message received from the network is needed.

DISCLOSURE OF THE INVENTION

The user equipment does not perform data transmission to an uplink eventhough the RRC connection is performed between the user equipment andthe network. For example, when a user performs Internet browsing, theuser does not take any action until it fully reads a desired web pagedownloaded from Internet. While the user does not take any action asabove, the user equipment performs an unnecessary effort (for example,continuous random access procedure) for maintaining uplinksynchronization, whereby radio resources or resources such as the powerof the user equipment may be wasted.

Accordingly, the present invention is directed to a method ofcontrolling uplink synchronization state at a user equipment in a mobilecommunication system, which substantially obviates one or more problemsdue to limitations and disadvantages of the related art.

An object of the present invention is to provide a method of controllinguplink synchronization state at a user equipment in a mobilecommunication system, in which the user equipment can efficientlycontrol a synchronization state with a network in the mobilecommunication system.

Another object of the present invention is to provide a method ofcontrolling uplink synchronization state at a user equipment in a mobilecommunication system, in which the user equipment can efficiently manageradio resources in accordance with shift of the synchronization state.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, amethod of controlling an uplink synchronization state at a userequipment in a mobile communication system comprises receiving controlinformation associated with controlling uplink synchronization state ofthe user equipment from a network, releasing uplink resources allocatedfrom the network if the uplink synchronization state is shifted from asynchronization state to an asynchronous state based on the controlinformation.

In another aspect of the present invention, a method of controlling anuplink synchronization state at a user equipment in a mobilecommunication system comprises receiving downlink data from a network,the downlink data including a dedicated random access preamble, andperforming a random access procedure using the dedicated random accesspreamble regardless of that the user equipment is in the uplinksynchronization state or asynchronous state.

In still another aspect of the present invention, a method ofcontrolling an uplink synchronization state at a user equipment in amobile communication system comprises being allocated with uplink ordownlink channel resources from a network, and releasing the uplink ordownlink channel resources if the user equipment is in an asynchronousstate; and performing a random access procedure with the network.

In further still another aspect of the present invention, a method ofcontrolling an uplink synchronization state at a user equipment in amobile communication system comprises being allocated with uplinkchannel resources from a network, receiving timing alignment commandfrom the network; driving a timing alignment timer, and releasing theallocated uplink channel resources if the timing alignment timer ends.

In further still another aspect of the present invention, a method ofcontrolling an uplink synchronization state at a user equipment in amobile communication system comprises receiving timing alignment commandfrom a network, driving a timing alignment timer for determining theuplink synchronization state, and performing a random access procedureif uplink data to be transmitted to the network occur in a state thatthe timing alignment timer ends.

According to the embodiments of the present invention, the userequipment can efficiently control a synchronization state with thenetwork in the mobile communication system, and can efficiently manageradio resources in accordance with the synchronization state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a structure of a physicalchannel used in a multiple carrier system according to the related art;

FIG. 2 is a diagram illustrating a network structure of an E-UMTS(Evolved Universal Mobile Telecommunications System);

FIG. 3 is a schematic view illustrating an E-UTRAN (Evolved UniversalTerrestrial Radio Access Network);

FIG. 4A and FIG. 4B are diagrams illustrating a structure of a radiointerface protocol between a user equipment (UE) and E-UTRAN, in whichFIG. 4A is a schematic view of a control plane protocol and FIG. 4B is aschematic view of a user plane protocol;

FIG. 5 is a flow chart illustrating a procedure according to oneembodiment of the present invention;

FIG. 6 is a flow chart illustrating a contention based random accessprocedure according to another embodiment of the present invention;

FIG. 7 is a flow chart illustrating a procedure according to anotherembodiment of the present invention; and

FIG. 8 is a flow chart illustrating a procedure according to otherembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, structures, operations, and other features of the presentinvention will be understood readily by the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Embodiments described later are examples in which technicalfeatures of the present invention are applied to an E-UMTS (EvolvedUniversal Mobile Telecommunications System).

FIG. 2 illustrates a network structure of an E-UMTS. An E-UMTS is asystem evolving from the conventional WCDMA UMTS and its basicstandardization is currently handled by the 3GPP (3^(rd) GenerationPartnership Project). The E-UMTS can also be called an LTE (Long TermEvolution) system. [http://www.3gpp.org/ftp/Specs/2006-12/] and[http://www.3gpp.org/ftp/Specs/html-info/GanttChart-Level-2.htm] can bereferred to, so as to obtain detailed information about technicalspecification of the UMTS and E-UMTS.

Referring to FIG. 2, an E-UTRAN includes base stations (hereinafter,referred to as ‘eNode B’ or ‘eNB’), wherein respective eNBs areconnected with each other through X2 interface. Also, each of eNBs isconnected with a user equipment (UE) through a radio interface andconnected with EPC (Evolved Packet Core) through S1 interface. The EPCincludes a mobility management entity/system architecture evolution(MME/SAE) gateway.

Layers of a radio interface protocol between a user equipment and anetwork can be classified into a first layer L1, a second layer L2 and athird layer L3 based on three lower layers of OSI (open systeminterconnection) standard model widely known in communication systems. Aphysical layer belonging to the first layer L1 provides an informationtransfer service using a physical channel. A radio resource control(hereinafter, abbreviated as ‘RRC’) located at the third layer plays arole in controlling radio resources between the user equipment and thenetwork. For this, the RRC layer enables RRC messages to be exchangedbetween the UE and the network. The RRC layer can be distributivelylocated at network nodes including Node B, an AG and the like or can beindependently located at either the Node B or the AG.

FIG. 3 is a schematic view illustrating an E-UTRAN (UMTS terrestrialradio access network). In FIG. 3, a hatching part represents functionalentities of a user plane, and a non-hatching part represents functionalentities of a control plane.

FIG. 4A and FIG. 4B illustrate a structure of a radio interface protocolbetween the user equipment (UE) and the E-UTRAN, in which FIG. 4A is aschematic view of a control plane protocol and FIG. 4B is a schematicview of a user plane protocol. Referring to FIG. 4A and FIG. 4B, a radiointerface protocol horizontally includes a physical layer, a data linklayer, and a network layer, and vertically includes a user plane fordata information transfer and a control plane for signaling transfer.The protocol layers in FIG. 4A and FIG. 4B can be classified into L1(first layer), L2 (second layer), and L3 (third layer) based on threelower layers of the open system interconnection (OSI) standard modelwidely known in the communications systems.

The physical layer as the first layer provides information transferservice to an upper layer using physical channels. The physical layer(PHY) is connected to a medium access control (hereinafter, abbreviatedas ‘MAC’) layer above the physical layer via transport channels. Dataare transferred between the medium access control layer and the physicallayer via the transport channels. Moreover, data are transferred betweendifferent physical layers, and more particularly, between one physicallayer of a transmitting side and the other physical layer of a receivingside via the physical channels. The physical channel of the E-UMTS ismodulated in accordance with an orthogonal frequency divisionmultiplexing (OFDM) scheme, and time and frequency are used as radioresources.

The medium access control (hereinafter, abbreviated as ‘MAC’) layer ofthe second layer provides a service to a radio link control(hereinafter, abbreviated as ‘RLC’) layer above the MAC layer vialogical channels. The RLC layer of the second layer supports reliabledata transfer. In order to effectively transmit data using IP packets(e.g., IPv4 or IPv6) within a radio-communication period having a narrowbandwidth, a PDCP layer of the second layer (L2) performs headercompression to reduce the size of unnecessary control information.

A radio resource control (hereinafter, abbreviated as ‘RRC’) layerlocated on a lowest part of the third layer is defined in the controlplane only and is associated with configuration, reconfiguration andrelease of radio bearers (hereinafter, abbreviated as ‘RBs’) to be incharge of controlling the logical, transport and physical channels. Inthis case, the RB means a service provided by the second layer for thedata transfer between the user equipment and the UTRAN.

As downlink transport channels carrying data from the network to theuser equipments, there are provided a broadcast channel (BCH) carryingsystem information, a paging channel (PCH) carrying paging message, anda downlink shared channel (SCH) carrying user traffic or controlmessages. The traffic or control messages of a downlink multicast orbroadcast service can be transmitted via the downlink SCH or anadditional downlink multicast channel (MCH). Meanwhile, as uplinktransport channels carrying data from the user equipments to thenetwork, there are provided a random access channel (RACH) carrying aninitial control message and an uplink shared channel (UL-SCH) carryinguser traffic or control message.

As logical channels located above the transport channels and mapped withthe transport channels, there are provided a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

In the E-UMTS system, an OFDM is used on the downlink and a singlecarrier frequency division multiple access (SC-FDMA) on the uplink. TheOFDM scheme using multiple carriers allocates resources by unit ofmultiple sub-carriers including a group of carriers and utilizes anorthogonal frequency division multiple access (OFDMA) as an accessscheme.

FIG. 5 is a flow chart illustrating a procedure according to oneembodiment of the present invention.

Referring to FIG. 5, the user equipment (UE) performs a random accessprocedure with the base station (eNB) through a random access channel(RACH) [S51]. Through the random access procedure, the user equipmentadjusts uplink synchronization and enters the synchronization state[S52].

FIG. 6 is a flow chart illustrating a contention based random accessprocedure according to another embodiment of the present invention.Referring to FIG. 6, if the MAC layer of the user equipment commands itsphysical layer to initiate the random access procedure, the physicallayer of the user equipment selects one access slot and one signature,transmits a random access preamble to the base station [S61]. If theuser equipment transmits the preamble, the base station transmits aresponse message through a downlink physical channel (for example, AICH(Acquisition Indicator Channel)) [S62]. The AICH transmitted in responseto the preamble transmits the signature selected by the preamble for afirst constant length of an access slot corresponding to the access slotthrough which the preamble has been transmitted. At this time, the basestation transmits ACK (acknowledgement) or NACK (non-acknowledgement) tothe user equipment through the signature transmitted from the AICH. Atthis time, the user equipment transmits a specific message using radioresource allocation information, message size, and radio parameters,which are included in the random access response message [S63]. Therandom access response message includes timing alignment information,and the user equipment acquires uplink synchronization using the timingalignment information.

If the user equipment receives NACK through the random access responsemessage, the MAC layer of the user equipment commands the physical layerof the user equipment to transmit the preamble again after a propertime. If the message is received from the user equipment, the basestation transmits a MAC contention resolution message to the userequipment [S64].

In FIG. 6, since the random access procedure is based on contention, theuser equipment cannot sure that the random access response message istransmitted to itself. Accordingly, the user equipment does notdetermine that it has acquired uplink synchronization until it clearlyidentifies successful random access by receiving the contentionresolution message. In other words, if it is identified through thecontention resolution message that the user equipment has failed inrandom access, the user equipment should determine that it has notacquired uplink synchronization.

Referring to FIG. 5 again, if the user equipment performs successfulrandom access, the user equipment enters the uplink (UL) synchronizationstate [S52]. The base station allocates uplink and/or downlink channelresources through uplink (UL)/downlink (DL) scheduling procedure so thatthe user equipment transmits uplink data or receives downlink data[S53].

The base station transmits control information to the user equipment,wherein the control information is to control the uplink synchronizationstate of the user equipment [S54]. In the embodiment of FIG. 5, thecontrol information includes time alignment command information. If thetime alignment command information is received, the user equipmentdrives a timer for time alignment. If the timer is already beingoperated when the user equipment receives the time alignment commandinformation, the user equipment drives the timer again.

If the timer ends without separate command related to the uplinksynchronization state of the user equipment from the base station, theuser equipment determines that it has been shifted to the uplinkasynchronous state [S55]. Therefore, the user equipment releases all theuplink or downlink channel resources allocated from the base station[S56]. In other words, the user equipment regards that the uplink ordownlink channel resources have not been allocated, and transmits uplinkdata through the uplink channel resources or does not try to receivedownlink data through the downlink channel resources. Also, if the userequipment needs to acquire uplink synchronization to transmit uplinkdata or due to other reason, it performs the random access procedure[S57].

FIG. 7 is a flow chart illustrating a procedure according to anotherembodiment of the present invention.

Referring to FIG. 7, the user equipment (UE) performs a random accessprocedure with the base station (eNB) through a random access channel(RACH) [S71]. Through the random access procedure, the user equipmentadjusts uplink synchronization and enters the synchronization state[S72]. The base station allocates uplink and/or downlink channelresources through uplink (UL)/downlink (DL) scheduling procedure so thatthe user equipment transmits uplink data or receives downlink data[S73]. Details of the above steps S71 to S73 can be obtained withreference to the description according to the embodiment of FIG. 5.

The base station transmits control information to the user equipment,wherein the control information is to control the uplink synchronizationstate of the user equipment [S74]. In the embodiment of FIG. 7, thecontrol information includes command information as to whether tocontinuously maintain the uplink synchronization state of the userequipment or whether to shift the uplink synchronization state of theuser equipment to the uplink asynchronous state. The user equipmentperforms a specific operation through communication with the basestation or performs its own operation in accordance with the controlinformation of the uplink synchronization state received from the basestation [S75]. The detailed operation performed by the user equipmentdepends on the control information, especially the command information,and will now be described in detail.

If the command information commands that the user equipment shouldmaintain the uplink synchronization state, the user equipment performsthe random access procedure periodically or non-periodically. At thistime, the base station can notify the user equipment of the periodrelated to performing the random access procedure. Alternatively, theuser equipment can determine the period or can perform the random accessprocedure non-periodically.

If the user equipment performs the random access procedure periodically,it drives a timer for measuring the period when the random accessprocedure successfully ends. If the timer for measuring the period ends,the user equipment performs the random access procedure again. If theuser equipment successfully transmits uplink data to the base stationeven before the timer ends, it drives the timer again. If the userequipment successfully performs the random access procedure or uplinkdata transmission, the user equipment regards that it is in the uplinksynchronization state. On the other hand, if the user equipment fails inthe random access or uplink data transmission, the user equipmentregards that it is in the uplink asynchronous state.

For another example, if the command information commands that the userequipment should maintain the uplink synchronization state, the basestation periodically allocates uplink radio resources to the userequipment so that the user equipment maintains the uplinksynchronization state. The user equipment transmits data or controlinformation to the base station using the allocated uplink radioresources. At this time, the user equipment can transmit an indicatortogether with the data or control information, wherein indicatorindicates that transmission of the data or control information is tomaintain uplink synchronization. The indicator can be transmitted withtogether with MAC PDU, for example, in addition to the data or controlinformation. The base station can transmit allocation information forallocating the uplink radio resources to the user equipment throughPDCCH on L1/L2 control channel. Alternatively, the base station cantransmit allocation information for allocating the uplink radioresources to the user equipment together with RRC message. In this case,the allocation information is included in the RRC message.

FIG. 8 is a flow chart illustrating a procedure according to otherembodiment of the present invention. The embodiment of FIG. 8 relates tothe operation of the user equipment if the user equipment is not surethat it is in the uplink synchronization state or the uplinkasynchronous state.

Referring to FIG. 8, the user equipment (UE) acquires uplinksynchronization through the random access procedure with the basestation (eNB) [S81]. The base station allocates a dedicated randomaccess preamble to the user equipment [S82]. The dedicated random accesspreamble can be allocated to the user equipment by the base station evenbefore the user equipment acquires the uplink synchronization. If agiven time passes without separate procedure after the user equipmentacquires the uplink synchronization state, the user equipment cannotidentify whether it is in the uplink synchronization state or the uplinkasynchronous state. In this case, the user equipment performs the randomaccess procedure with the base station using the dedicated random accesspreamble regardless of that it is in the uplink synchronization state orthe uplink asynchronous state, and then acquires the uplinksynchronization [S83]. In other words, except that the dedicated randomaccess preamble is allocated through RRC message for handover, the userequipment regards that it is in the uplink asynchronous state.

In FIG. 8, in a state that the user equipment is in the uplinkasynchronous state [S84], the user equipment is allocated with uplink ordownlink channel resources from the base station [S85]. In this case,the user equipment does not regard that it is in the uplinksynchronization state, with only the fact that it has been allocatedwith the uplink or downlink channel resources. Accordingly, the userequipment does not perform data transmission through the allocateduplink channel resources, and does not transmit ACK/NACK in response tothe allocated downlink channel resources, either. Also, the userequipment performs the random access procedure with the base station. Inthis case, the user equipment notifies the base station that errorrelated to the uplink synchronization has occurred. In other words, theuser equipment notifies the base station that it has been allocated withuplink or downlink radio resources in a state that it is in the uplinkasynchronous state.

According to another embodiment of the present invention, the userequipment can manage the uplink synchronization state in accordance witha given state. Hereinafter, detailed examples of managing the uplinksynchronization state will be described.

According to the first example, if a predetermined condition which ispreviously set is satisfied, the user equipment periodically performsthe random access procedure or transmits data through the uplink tomaintain the uplink synchronization state. The predetermined conditionrelates to whether signal quality of a cell where the user equipment islocated is greater than or less than a given level. Alternatively, thepredetermined condition relates to whether signal quality of aperipheral cell of the cell where the user equipment is located isgreater than or less than a given level.

According to the second example, if the uplink channel resources areallocated to the user equipment, the user equipment regards that thebase station has commanded the user equipment to maintain the uplinksynchronization state, and periodically performs the random accessprocedure or transmits data to the base station through the uplink. Inother words, if the dedicated uplink channel resources are allocated tothe user equipment, the user equipment performs the operation formaintaining the uplink synchronization state. Also, if the base stationreleases the dedicated radio resources allocated to the user equipment,the user equipment regards that the base station has commanded the userequipment to shift the current state to the uplink asynchronous state.In this case, the user equipment does not perform the operation formaintaining the uplink synchronization state.

If the user equipment is shifted to the uplink asynchronous state in astate that the uplink radio resources are allocated to the userequipment, the user equipment releases its dedicated uplink radioresources. In other words, the user equipment does not regard that theuplink radio resources are allocated to the user equipment, and does notperform data transmission using the uplink radio resources. The userequipment transmits uplink data using its dedicated uplink radioresources only if it is in the uplink synchronization state. Forexample, the dedicated uplink radio resources are to transmit soundingreference signal (SRS) or CQI. For another example, the uplink radioresources could be radio resources for transmitting ACK/NACK topersistently scheduled resources set to transmit data through the uplinkor another persistently scheduled resources set to receive datatransmitted through the downlink. For other example, the uplink radioresources mean dedicated scheduling request channels used to allow theuser equipment to request the base station of radio resources.

The aforementioned embodiments are achieved by combination of structuralelements and features of the present invention in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment. Moreover, it will be apparent that someclaims referring to specific claims may be combined with another claimsreferring to the other claims other than the specific claims toconstitute the embodiment or add new claims by means of amendment afterthe application is filed.

The embodiments according to the present invention may be implemented byvarious means, for example, hardware, firmware, software, or theircombination. If the embodiment according to the present invention isimplemented by hardware, the embodiment of the present invention may beimplemented by one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, microcontrollers,microprocessors, etc.

If the embodiment according to the present invention is implemented byfirmware or software, the method of transmitting and receiving data inthe wireless communication system according to the embodiment of thepresent invention may be implemented by a type of a module, a procedure,or a function, which performs functions or operations described asabove. A software code may be stored in a memory unit and then may bedriven by a processor. The memory unit may be located inside or outsidethe processor to transmit and receive data to and from the processorthrough various means which are well known.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

INDUSTRIAL APPLICABILITY

The present invention can be used in a wireless communication systemsuch as a mobile communication system or a wireless Internet system.

1. (canceled)
 2. A method of maintaining time alignment of a userequipment (UE) in a mobile communication system by performing randomaccess procedures, the method comprising: transmitting a random accesschannel (RACH) preamble from the user equipment to a network to performa random access procedure; receiving, by the UE, a random access channel(RACH) response from the network, wherein the RACH response comprises atiming command to enter into a synchronization state; starting, by theUE, a time alignment timer for uplink time alignment when the timingcommand is received from the network, wherein the time alignment timeris for controlling an uplink synchronization state of the UE; andreceiving, by the UE, a contention resolution message from the networkto determine whether the random access procedure is successful or not,wherein the UE considers itself as in an asynchronous state if the UEdetermines that the random access procedure is not successful based onthe contention resolution message.
 3. The method of claim 2, wherein theUE considers itself as in a synchronous state if the UE determines thatthe random access procedure is successful based on the contentionresolution message.
 4. The method of claim 2, further comprisingperforming a random access procedure to enter into the uplinksynchronization state.
 5. The method of claim 2, further comprisingperiodically performing a random access procedure with the network. 6.The method of claim 3, further comprising: receiving a messageallocating uplink radio resources from the network; and transmittinguplink data to the network through the allocated uplink radio resources.7. The method of claim 3, wherein the uplink data includes informationindicating that the data transmission is to maintain the uplinksynchronization state.
 8. A user equipment (UE) for maintaining timealignment in a mobile communication system by performing random accessprocedures, the user equipment configured to: transmit a random accesschannel (RACH) preamble to a network to perform a random accessprocedure; receive a random access channel (RACH) response from thenetwork, wherein the RACH response comprises a timing command to enterinto a synchronization state; start a time alignment timer for uplinktime alignment when the timing command is received from the network,wherein the time alignment timer is for controlling an uplinksynchronization state of the UE; and receive a contention resolutionmessage from the network to determine whether the random accessprocedure is successful or not, wherein the UE considers itself as in anasynchronous state if the UE determines that the random access procedureis not successful based on the contention resolution message.
 9. Theuser equipment of claim 8, wherein the UE considers itself as in asynchronous state if the UE determines that the random access procedureis successful based on the contention resolution message.
 10. The userequipment of claim 8, wherein the UE is further configured to perform arandom access procedure to enter into the uplink synchronization state.11. The user equipment of claim 8, wherein the UE is further configuredto periodically perform a random access procedure with the network. 12.The user equipment of claim 9, wherein the UE is further configured to:receive a message allocating uplink radio resources from the network;and transmit uplink data to the network through the allocated uplinkradio resources.
 13. The user equipment of claim 9, wherein the uplinkdata includes information indicating that the data transmission is tomaintain the uplink synchronization state.